Interpretive Summary: Forty-eight states in the U.S. have decided to use phosphorus (P) Indices to meet the requirements of their NRCS Code 590 Standard. The majority of the states chose to develop these Indices without consultation or coordination with neighboring states in order to meet specific local conditions and policy needs. In this study we compared output from P Indices from 12 southern states against P runoff data collected from field plots in Georgia and two watersheds in North Carolina. This study demonstrated that while many of the P Indices were strongly correlated with the measured water quality data, there exists a large amount of variability among the southern P Indices that may result in different P management strategies being employed under similar conditions.

Technical Abstract:
Forty-eight states in the U.S. have decided to use phosphorus (P) Indices to meet the requirements of their NRCS Code 590 Standard. The majority of the states chose to develop these Indices without consultation or coordination with neighboring states in order to meet specific local conditions and policy needs. Using data collected from two southern agricultural experiments (GA and NC), we compared P losses with P-Index ratings using the 12 southern P Indices. Measured phosphorus losses were 2 to 10 times greater from the Georgia plots (5.8 to 32.8 kg TP ha-1) than the North Carolina watersheds (2.75 kg TP ha-1 and 5.30 kg TP ha-1), due in part to differences in scale (plot vs watershed). Phosphorus-Index ratings for the Georgia plots ranged from low to very high, with only five Indices (AR, FL, GA, and TN) identifying very high losses from these plots most of the time. Conversely, nine Indices (AL, AR, FL, GA, LA, OK, SC, TN, and TX) produced ratings from Very High to Severe for one of the two North Carolina watersheds. When P loss was regressed with P-Index ratings, moderate to very strong relationships existed for nine Indices (AR, SC, MS, NC, TN, GA, LA, and FL) and all but one Index was directionally correct. This study demonstrated that while many of the P Indices were directionally and magnitudinally correct relative to the measured water quality data, there exists a large amount of variability among the southern P Indices that may result in different P management strategies being employed under similar conditions.